The present invention relates to a burner for a gas turbine or the like, and more particularly, to a premix burner in which a fuel is mixed with air before the fuel is burned.
As disclosed by Publications of Japanese Patent Unexamined Publication Nos. 64-54122 and 2-40418, a burner has generally a two-steps burner system for decreasing a density of NO.sub.x so that the fuel is mixed with the air prior to burning of the fuel, when a rated output is obtained. The fuel is mixed with the air prior to burning of the fuel in a second burner by a premix swirler of a premixing device. In order to decrease the density of NO.sub.x, an even mixing of the fuel-and-air and a low density of fuel in the mixture are effective. Therefore, the mixing of the fuel-and-air proceeds in a large space and the premix swirl in the premix device accelerates the even mixing. An eddy generating device or flow obstructing member, that is, a flame keeper is arranged near a downstream side of the premix swirler as disclosed in Publication of Japanese Patent Unexamined No. 64-54122 or is movable longitudinally at the downstream side of the premix swirler according to a variation of temperature in the burner as disclosed in of Japanese Patent Unexamined Publication No. 2-40418, and the swirl extends in the premix device to mix the fuel and the air between upstream and downstream sides of the premix device. In the conventional premix burner, a flame is formed and extinguished alternately at a downstream end of the premix swirler so that a vibration is generated in the premix burner and an operation of the premix burner is not stable.